Within packaging technology what are called aseptic packagings are often used for packaging and transporting products of a type sensitive to oxygen and/or light. This may apply to Liquid foodstuffs such as juice, wine, edible oil and medical/pharmaceutical/biological preparations which would otherwise easily be damaged or deteriorate already after very short exposure to oxygen and/or light. Aseptic packagings are also used for other products such as milk in order to give the packaged product an extended shelf life so that it can be kept with maintained freshness qualities for very long periods of time without the need for refrigerated storage.
The requirement that is set for aseptic packagings is that they must be oxygen-tight/lightproof and in addition bacteria-tight in order to give the best possible protection to the packaged product. Further requirements for the packaging are that it must be form stable and mechanically strong in order to be able to withstand external stresses to which the packaging is exposed during normal transport and handling. It is naturally an advantage if the packaging is in addition easy to manufacture and after use easy to destroy or to be handled in another manner with fulfilment of current environmental protection requirements.
A large group of known aseptic packagings is manufactured from a laminated material comprising on the one hand a strengthening skeletal layer of paper or cardboard, on the other hand a barrier layer of aluminium (Al foil), which gives the packaging the desired impermeability properties, applied to one side of the skeletal layer, and also further coatings of plastic (usually polythene) to make the material easy to seal through what is known as heat sealing.
From the known packaging material aseptic packagings of disposable character are manufactured, most frequently with the aid of modern rational packaging machines of the type which, either from a strip or from a prefabricated substance of the material, both forms, fills and closes the packagings on a large industrial scale with a high rate of production. Well known examples of aseptic packagings of this type are Tetra Brik (reg. trade mark) and Tetra Rex (reg. trade mark).
A packaging of the type Tetra Brik (reg. trade mark) is manufactured from a strip through the strip first being shaped to a tube through the two longitudinal sides of the strip being joined to each other in a longitudinal overlap joint. The tube is filled with the contents in question and divided into closed cushion-shaped packaging units through repeated transverse sealings of tube zones across the longitudinal direction of the tube below the filling level of the tube. The packaging units are separated from each other through cuts in the transverse sealing zones and given the desired geometric, usually parallelepiped shaped final form in a concluding shaping operation during which the double-walled triangular corner flaps of the packagings are folded against and sealed to an adjacent side of the packaging.
The known packaging material has several serious disadvantages which to a great extent and in certain cases can be wholly ascribed to the Al foil used as a barrier layer, which owing to its low elasticity often breaks in particularly stressed areas of the material during manufacture of the packaging and thereby causes deterioration in the hermetic property of the finished packaging. An Al foil is in addition expensive to produce and therefore contributes markedly to giving the packaging high material costs. The known packaging material also has disadvantages owing to the absorbent layer of paper or cardboard which quickly loses its mechanical strength properties and makes the packaging limp and unmanageable when it is exposed to liquids and moisture. The layer of paper or cardboard must therefore be made relatively thick in order to give the packaging the necessary rigidity of form and stability of form, which contributes to increasing the material stress and therewith the risk of formation of cracks in the Al foil during manufacture of the packaging.
Within packaging technology it has therefore always been an objective to find alternative packaging materials which neither use a fibre layer nor Al foil and which therefore are in practice completely freed from drawbacks of the type inherent in the known paper or cardboard based packaging materials.
Such an alternative packaging material which at least partly avoids the problems mentioned here is described for example in EP-A-O353 991. This known material has a strengthening skeletal layer of plastic and filler mixed in the plastic and has proved to be less costly and less moisture sensitive than the paper or cardboard based material. A similar packaging material is also described in EP-A-O353 496.
A packaging material consisting only of plastic and filler mixed in the plastic, as described in both these European patent applications, nevertheless lacks the necessary hermetic properties against both oxygen and light and must therefore be supplemented with at least one further layer of a material with the desired hermetic properties in order to be able to be used in aseptic packagings as intended. The only example of such a supplementary layer that is mentioned in both these patent applications is, however, the already discussed expensive Al foil which is prone to cracking.
Swedish patent application no. 9100057-0 (which corresponds to U.S. application Ser. No. 07/818,156) describes a packaging material which, like the two last mentioned packaging materials described above, has a strengthening skeletal layer of plastic and filler mixed in the plastic but which, instead of an Al foil, uses an oxygen barrier layer consisting of plastic of the same type as the plastic in the skeletal layer in a mixture with plastic of a different type from the plastic in the skeletal layer. As a particularly preferred type the Swedish application patent no. 9100057-0 (which corresponds to U.S. application Ser. No. 07/818,156) describes a packaging material in which the plastic in the skeletal layer is to consist of a propylene homopolymer with a melting index of less than 1 according to ASTM (2.16 kg; 230.degree. C.) or an ethylene/propylene copolymer with a melting index between 0.5 and 5 according to ASTM (2.16 kg; 230.degree. C.) and an ethylene/vinyl alcohol copolymer.
This last described packaging material can, however, be further improved and the object of the present invention is therefore to give indications of how such improvements can be effected.